1. Field
The subject invention is in the field of lanyards, tethers and leashes, particularly leashes. Also, it is in the field of self retractable tapes and leashes. Specifically, it is in the field of retractable leashes and, more specifically, self retractable leashes for large, fast dogs.
2. Prior Art
The prior art in this specific field and known to the inventor of the subject invention is a commercially available retractable leash which has a casing including a hand grip and enclosing a reel for a line which is the leash line. A spring causes the reel to wind the line in when tension in the line allows. A button convenient to the holder's thumb is depressed to engage teeth on the spring powered reel to, when desired, prevent it from turning. The leash is available in a range of sizes and a range of leash line lengths for each size.
Such leashes are highly useful in communities in which the law requires that dogs be on a leash when not on the owner's property or otherwise confined. The leashes allow the dogs the freedom provided by a long leash and eliminate the need for the user to repeatedly handle the leash line, pulling it in to prevent it from becoming entangled with the dogs legs, trees, bushes, etc. and alternately letting it out again. The reel locking feature provides the control needed to limit the range when necessary.
It is generally recognized that the larger the dog, the longer the leash line required to take optimum advantage of the automatic rewinding leash. For optimum use it is also required that the rewinding speed be faster in proportion to increase of dog size since the speeds of dogs are generally related to their sizes. For a range of tension in the line comfortable for the user, the two requirements noted above are technically incompatible. The longer the line, the more mass there is to be accelerated by the reel spring when the leash is shortened and therefore the more tension needed to achieve a given retraction speed. With higher speed retraction it is also necessary that the acceleration to that speed be increased. The formula showing the relationship among force (tension), mass and acceleration is F=ma in which F is the tension, m is the mass of the line and a is the acceleration possible under the circumstances. The expression for a from this equation is a=F/m. From this equation it can be seen that acceleration decreases with increases in the mass, i.e. increases in the length of the line. To double the retraction speed using one reel and spring the spring force (tension) would have to be increased four times, two to handle the doubled mass and two to double the acceleration needed to reach the doubled speed.
The subject invention addresses the need for increasing both the range and the retraction speed of leash apparatus for animals relative to the tension level in the leash line. The primary objective of the subject invention is to meet this requirement. A second objective is that the equipment used in meeting the requirement be such that the cost of the leash apparatus would not inhibit its use.